The present invention relates to a steel plate showing good formability in the forming stage and yet providing for excellent strength in use. More particularly, the invention relates to a highly formable steel plate which can be enhanced in strength in necessary areas by high-density energy treatment after forming or a steel plate which has been enhanced in strength in preselected not-severely-forming areas by said high-density energy treatment and can, therefore, be easily formed. In the following description of the invention, the post-forming laser treatment mode of the invention will be chiefly described but as pointed out above, the laser treatment according to the invention can be performed prior to forming as well. Similarly, the application of the invention to automotive body members will be described as a typical application but the scope of the invention is not limited to such particular application but covers a variety of applications demanding the above-mentioned two requirements, viz. formability and increased strength.
Automotive parts, particularly body members, are required to satisfy two conflicting requirements, viz. ease of forming and high strength. Thus, such members must have high formability in order that they may fit neatly to the streamlined contour of a car body and, at the same time, should have been highly increased in strength in strategical areas so that adequate protection may be afforded to the passenger in the event of, for example, a collision on the road. Therefore, the technology of press-forming a highly formable low-carbon steel blank and increasing its strength in predetermined regions with a high-density energy source has been proposed (cf. Japanese Tokkyo Kokai Koho S-61-99629). However, when such a blank is irradiated using a high-density energy source, for example a laser, under the conditions described in the patent specification referred to above, an uneven penetration of heat across the thickness of the plate tends to cause a strain, thus necessitating reshaping following laser treatment. Moreover, the required number of laser scan lines is considerably increased to cause a practically unacceptable protraction of treating time.
This technology based on the concept of laser hardening after press-forming is such that a blank is first press-formed in a press line and then exposed to a high-density energy but the research so far undertaken has generated no information at all about what is the optimum combination of material steel microstructure and high-density energy treatment parameters that would minimize said strain or whether such combination would lead to a sufficient enhancement of strength. Therefore, a great demand exists for the generation of information on the optimum combination of high-density energy treatment parameters and steel microstructure. Thus, the development, based on the knowledge of steel microstructure, of a steel blank which would be easily formable in the press-forming stage and could then be enhanced in strength after forming has been awaited.
Aside from the above technology, Japanese Tokkyo Kokai Koho H-4-72010 discloses a process comprising exposing a press-formed member to laser light to achieve an enhancement of strength. This patent specification states that such increases in strength can be obtained by subjecting carbon steel plate to laser treatment. However, as regards the composition of steel, this prior art refers only to the amount of carbon and does not refer to alloying elements other than carbon, nor does it refer to the microstructure of steel. Therefore, no information is available from this literature on the correlation of alloying elements and steel microstructure with laser treatment parameters. The research done by the inventors of the present invention revealed that the enhancement of strength due to laser treatment is dependent not only on laser parameters but also, significantly, on the alloying elements and microstructure of steel. Therefore, in order to realize a useful increase in strength by laser treatment, it was considered essential to elucidate the above-mentioned correlation.
In this connection, Japanese Tokkyo Kokai Koho S-61-261462 provides some information on a formable steel plate for laser treatment use but the formability discussed there is the press-formability of laser-cut steel. In contrast, the present invention is directed to laser hardening and although the same term `laser treatment` is used, the invention is quite different from the above technology in that it is not directed to steel cutting.
Furthermore, Japanese Tokkyo Kokai Koho H-1-259118 discloses a technology for achieving an increase steel strength which comprises subjecting strength-required zones of a press-formed material to rapid remelting-rapid solidification treatment to locally induce formation of microfine crystal grains. However, this laid-open patent specification is directed to a selective melting of the zone which would constitute the reverse side in use and unlike the through-melting technology of the present invention, it produces a large residual strain and, moreover, does not provide a sufficient increase in strength. Moreover, the mechanism of strength enhancement in the above technology resides in a decreased size of crystal grains and not in hardening. In this respect, too, this prior art technology should be differentiated from the present invention whose mechanism is concerned with the formation of a hardened microstructure.
Still further Japanese Tokkyo Kokai Koho S-57-70238 discloses a method of hardening treatment, but does not refer to chemical composition of the mother steel.
It is, therefore, clear that the hitherto-known processes are fundamentally different from the process of the invention which is described in detail herein-after.